Olivine-type cathode material is a promising candidate for use in batteries, especially olivine-type lithium metal phosphate materials such as LiFePO4. Its phosphate-containing structure provides it with good electrochemical stability and very good cyclability, however, its electronic conductivity is poor. To control its particle size, insure good surface crystallinity, and improve its conductivity, the material is generally coated with carbon. Another problem associated with LiFePO4 relates to its storage which needs to be under controlled environment to avoid its oxidation. Olivine-type materials and especially LiFePO4 are partially and rapidly oxidized when in contact with humidity. Consequently, ensuring the good stability of the powder when exposed to ambient air, for instance, under industrial conditions used for electrode fabrication, is a real challenge.
Reduction of diazonium salts has been investigated in the last decades for the functionalization of surfaces (see D. Belanger et al., Chem. Soc. Rev., 2011, 40, 3995-4048; M. Delamar et al., J. Am. Chem., Soc. 1992, 114, 5883-5884; and M. Toupin et al., J. Phys. Chem. C, 2007, 111, 5394-5401), mainly including the modification of carbon material surfaces. Modified carbon surfaces may be prepared by reaction with a diazonium salt in a liquid medium to attach organic groups on the surface.
Modification is possible, for instance, by reaction of the substrate with diazonium ions generated in situ (see U.S. Pat. Nos. 5,672,198; 5,698,016; and 5,707,432). A substituted aromatic amine, in the presence of a diazotizing agent in solution, leads to the formation of the corresponding diazonium cation. The chemical grafting process leads to the attachment of different substituted aryl groups including a strong covalent carbon-carbon bond between the substrate and the grafted group. Grafting using the diazonium chemistry is generally accepted to occur via a radical mechanism (F. Barrière et al., J. Solid State Electrochem., 2008, 12, 1231-1244). The resulting organic radical reacts with the surface and leads to a covalent bond (S. Mahouche-Chergui et al., Chem. Soc. Rev., 2011, 40, 4143-4166). However, the functionalization mechanism for spontaneous grafting is still unclear (M. Toupin et al., Langmuir, 2008, 24, 1910-1917; F. Le Floch et al., Electrochim. Acta, 2009, 54, 3078-3085; A. Adenier et al., Surface Science, 2006, 600, 4801-4812; P. Abiman et al., J. Phys. Org. Chem., 2008, 21, 433-439; and A. Mesnage et al., Langmuir, 2012, 28, 11767-11778. Nevertheless, previous studies have demonstrated that the layer could grow through radical reactions in the ortho position of the functional group (F. Barrière et al., Supra).
Diazonium-based functionalization has been first proposed for the stabilization of the carbon electrode of a Li-battery by formation of a lithium benzoate layer (see L. G. Shaidarova et al., J. Anal. Chem., 2008, 63, 922-942) or nitrophenyl layer (see D. M. A. I. Turyan, Electroanalysis, 1996, 8, 207-213). Silicon anodes modified by organic molecules derived from diazonium ions have shown a superior cycling stability (see S. Yang et al., J. Mater. Chem., 2012, 22, 3420; S. Yang et al., Electrochem. Comm., 2010, 12, 479-482; C. Martin et al., Adv. Funct. Mater., 2011, 21, 3524-3530; C. Martin et al., Adv. Mater., 2009, 21, pp. 4735-4741). The initial study on diazonium modified positive electrode materials involved the grafting of nitrophenyl groups on Li1.1V3O8 (F. Tanguy et al., J. Mater. Chem., 2009, 19, 4771). Later, LiFePO4 powders have been functionalized with redox molecules in order to assist the insertion of Li+ ions (see L. Madec et al., J. Pow. Sour., 2013, 232, 246-253). The spontaneous grafting of nitrophenyl groups at the surface of C—LiFePO4 was attempted but partial oxidation of LiFePO4 was observed, thus leading to the formation of a delithiated phase (see L. Madec et al., J. Am. Chem. Soc., 2013, 135, 11614-11622).
Electrochemical grafting of 4-trifluoromethylbenzene groups was previously carried out on copper or glassy carbon electrodes (see M. Weissmann et al., Carbon, 2010, 48, 2755-2764; and G. Shul et al., ACS Appl. Mater. Interfaces, 2013, 5, 1468-1473). A Vulcan XC72 carbon powder typically used for the preparation of PEMFC Pt/C type active layer was also chemically modified by spontaneous grafting of 4-trifluoromethylbenzene groups. However, the grafting of 6.5 wt. % of these molecules significantly affected the macroscopic behavior of the carbon powder. U.S. Pat. No. 6,399,202 describes a process for the preparation of modified carbon black using 3-trifluoromethylaniline as the starting molecule.